The antitumor agent BCNU was studied in order to determine its effect on thymic and splenic T and B lymphocytes in normal and immunosuppressed mice. An initial temporary decline in the PHA and LPS-sensitive splenic lymphocytes of BCNU-treated mice was detected. The PHA-sensitive thymic lymphocytes of BCNU-treated mice were totally depleted by the tenth day. The immunostimulator pyran and the antitumor agent BCNU were studied to determine their effect on T and B lymphocytes of tumor-bearing mice. The tumor-bearing group treated with BCNU had T and B cells lower than the tumor controls. The administration of pyran resulted in an increase of T cells and a temporary increase in B cells. These experimental results indicate that pyran reconstitutes the depressed T cell population and has a transitory effect on B cells resulting from tumor burden. Studies were conducted to determine the effect of pyran on the T and B cell populations of tumor bearing mice exposed to 500 rads of whole body irradiation. Pyran was found to be relatively effective in reconstituting the T but not B cell compartment of these tumored irradiated mice.